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surface-rupturing earthquakes since the           ongoing tectonic strain in the forearc of the            Bulletin, v. 114, no. 2, p. 169–177, doi: 10.1130/
formation of the surface ca. 15 ka. For a         active Cascadia subduction zone.                         0016-7606(2002)114<0169:LSASOT>2.0.CO;2.
60–90° reverse fault, the displacements                                                                 Blakely, R.J., Sherrod, B.L., Weaver, C.S., Wells,
across the scarp require minimum dip              IMPLICATIONS FOR SEISMIC                                 R.E., and Rohay, A.C., 2014, The Wallula fault
displacements of 6.4 ± 1.5 m for inter-           HAZARD                                                   and tectonic framework of south-central
fluves (n = 8) and 4.4 ± 1.1 m for channels                                                                Washington, as interpreted from magnetic and
(n = 4). The ~2 m difference in displace-           The length of the active Leech River                   gravity anomalies: Tectonophysics, v. 624–625,
ment between the channels and interfluves         fault zone (30–60 km; Fig. 2A) and its                   p. 32–45, doi: 10.1016/j.tecto.2013.11.006.
implies multiple episodes of fault activity       history of multiple Quaternary ruptures               Blyth, H., and Rutter, N., 1993, Surficial geology
and suggests that at least one event with         suggest it is capable of producing earth-                of the Sooke area (nts 92b/5): British Columbia
~2 m displacement occurred after the for-         quakes of MW >6. This active fault zone                  Ministry of Energy Mines and Petroleum
mation of the colluvial apron but before          lies within tens of kilometers of down-                  Resources, Open File 1993-25.
channel incision. In addition to this early       town Victoria and in close proximity to               Cassidy, J.F., Rogers, G.C., and Waldhauser, F.,
event, the ~4 m of displacement of the            three local water dams. One of these dams                2000, Characterization of active faulting
channels (Fig. 3B and DR2 [see footnote           is located within 2 km of the active fault               beneath the Strait of Georgia, British Columbia:
1]) requires either one large event with          zone and supports the region’s principal                 Bulletin of the Seismological Society of
~4 m of slip, or multiple smaller events          water supply reservoir (Fig. 2A). The other              America, v. 90, no. 5, p. 1188–1199, doi: 10.1785/
that together sum to ~4 m of slip. Global         two dams lie within the active fault zone                0120000044.
empirical relationships suggest that sur-         and support a hydroelectric power plant.              Clague, J.J., and James, T.S., 2002, History and
face displacements on the order of meters         Thus, our new identification of a signifi-               isostatic effects of the last ice sheet in southern
correspond to earthquakes of MW 6 or              cant shallow seismic source has consider-                British Columbia: Quaternary Science Reviews,
greater (e.g., Wells and Coppersmith, 1994).      able implications for the seismic risk                   v. 21, no. 1–3, p. 71–87, doi: 10.1016/S0277-3791
                                                  exposure of this populated region.                       (01)00070-1.
IMPLICATIONS FOR REGIONAL                         Surface-rupturing earthquakes with shal-              Fairchild, L., and Cowan, D., 1982, Structure,
SEISMOTECTONICS                                   low hypocenters can be highly destruc-                   petrology, and tectonic history of the Leech
                                                  tive, and it is therefore important that the             River complex northwest of Victoria, Vancouver
  Several observations indicate the active        Leech River fault zone be incorporated                   Island: Canadian Journal of Earth Sciences,
Leech River fault zone is part of a network       into seismic hazard assessments of south-                v. 19, p. 1817–1835, doi: 10.1139/e82-161.
of high-angle oblique faults that accom­          western British Columbia and neighboring              Gledhill, K., Ristau, J., Reyners, M., Fry, B., and
modate regional transpression across the          regions.                                                 Holden, C., 2011, The Darfield (Canterbury,
Juan de Fuca Strait and Puget Sound region.                                                                New Zealand) MW 7.1 earthquake of September
Barrie and Greene (2015) trace the Devil’s        ACKNOWLEDGMENTS                                          2010: A preliminary seismological report:
Mountain fault of Washington state, USA,                                                                   Seismological Research Letters, v. 82, no. 3,
to within 10–20 km of the fault scarps on            We thank CRD watersheds and BC Hydro for              p. 378–386, doi: 10.1785/gssrl.82.3.378.
Fig. 2, and their bathymetric and seismic         access to key field sites. This manuscript benefit-   Hetzel, R., and Hampel, A., 2005, Slip rate variations
surveys reveal a steeply dipping oblique          ted from comments by Jack Loveless, Alan Nelson,         on normal faults during glacial–interglacial
-slip fault zone similar to our observations      Christie Rowe, and two anonymous reviewers.              changes in surface loads: Nature, v. 435, 7038,
of the Leech River fault zone. Both the           We thank Steven Whitmeyer for editorial han-             p. 81–84, doi: 10.1038/nature03562.
Darrington–Devil’s Mountain fault and             dling. This research was supported by an NSERC        James, T., Bednarski, J., Rogers, G., and Currie, R.,
the Southern Whidbey Island fault systems         Discovery grant to KM and NSF EAR IRFP Grant             2010, LiDAR and digital aerial imagery of the
of Washington state (Fig. 1) are likewise         #1349586 to CR.                                          Leech River Fault Zone and coastal regions
near-vertical fault zones with oblique slip                                                                from Sombrio Point to Ten Mile Point, southern
histories (Sherrod et al., 2008; Personius et     REFERENCES CITED                                         Vancouver Island, British Columbia: Geological
al., 2014) similar to many of the crustal                                                                  Survey of Canada, Open-File 6211, doi: 10.4095/
fault systems throughout the Puget Sound          Balfour, N., Cassidy, J., Dosso, S., and Mazzotti,       285486.
region (e.g., McCaffrey and Goldfinger,              S., 2011, Mapping crustal stress and strain in     Johnson, S., Potter, C., Miller, J., Armentrout, J.,
1995; ten Brink et al., 2006; Blakely et al.,        southwest British Columbia: Journal of                Finn, C., and Weaver, C., 1996, The southern
2014; Nelson et al., 2014; Sherrod et al.,           Geophysical Research: Solid Earth (1978–2012),        Whidbey Island fault: An active structure in the
2016). Considering these similarities in             v. 116, B03314, doi: 10.1029/2010JB008003.            Puget Lowland, Washington: GSA Bulletin,
orientation and slip sense, we suggest that                                                                v. 108, p. 334–354, doi: 10.1130/0016-7606(1996)
the Leech River fault is part of this regional    Balk, D., Deichmann, U., Yetman, G., Pozzi, F.,          108<0334:TSWIFA>2.3.CO;2.
active forearc fault system. Although it             Hay, S., and Nelson, A., 2006, Determining         Johnson, S., Dadisman, S., Mosher, D., Blakely,
remains possible that the timing of past             global population distribution: Methods,              R., and Childs, J., 2001, Active tectonics of the
ruptures along these fault systems was               applications and data: Advances in Parasitology,      Devil’s Mountain Fault and related structures,
influenced by stress loading or release              v. 62, p. 119–156, doi: 10.1016/S0065-308X(05)        Northern Puget Lowland and Eastern Strait of
related to the last glaciation (e.g., Hetzel and     62004-0.                                              Juan de Fuca Region, Pacific Northwest: U.S.
Hampel, 2005), repeated earthquakes on                                                                     Geological Survey Professional Paper 1643.
crustal faults including the Leech River          Barrie, V., and Greene, G., 2015, Active faulting in  Johnston, S.T., and Acton, S., 2003, The Eocene
should be expected in order to accommodate           the northern Juan de Fuca Strait, Implications        Southern Vancouver Island Orocline: A response
                                                     for Victoria, British Columbia: Geological            to seamount accretion and the cause of fold-and-
                                                     Survey of Canada Current Research 2015-6,             thrust belt and extensional basin formation:
                                                     p. 10, doi: 10.4095/296564.                           Tectonophysics, v. 365, no. 1–4, p. 165–183,
                                                                                                           doi: 10.1016/S0040-1951(03)00021-0.
                                                  Blakely, R.J., Wells, R.E., Weaver, C.S., and         Johnson, S.Y., Dadisman, S.V., Childs, J.R., and
                                                     Johnson, S.Y., 2002, Location, structure, and         Stanley, W.D., 1999, Active tectonics of the
                                                     seismicity of the Seattle fault zone, Washington:     Seattle fault and central Puget Sound,
                                                     Evidence from aeromagnetic anomalies, geologic        Washington—Implications for earthquake hazards:
                                                     mapping, and seismic-reflection data: GSA             GSA Bulletin, v. 111, no. 7, p. 1042–1053,
                                                                                                           doi: 10.1130/0016-7606(1999)111<1042:ATOT
                                                                                                           SF>2.3.CO;2.

                                                  www.geosociety.org/gsatoday                                                                                   9
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